This invention relates generally to satellite communication systems and methods. It specifically relates to such systems and methods that enable real-time high bandwidth communications to occur between a remote location and a public access information resource, such as the Internet. This invention relates to both geostationary satellites (GEO's) and non-geostationary satellites (LEOs).
Geostationary Satellites (GEO's)
Satellite communication systems have been used as a high-speed pipeline sending many signals in one direction. These signals include various information or data such as video signals or telephone signals, typically compressed at one site and transmitted to a second site or to multiple sites and decompressed.
In satellite communication systems, a concept called Very Small Aperture Terminal (VSAT) permits multiple locations to send small packets of data to other locations via the satellite "pipelines." The predominate transmission technique uses polling, in which a ground station hub uses a satellite transmission to poll one or more remote sites for any data the remote sites have to send back to the hub. Data can be sent to the hub, analyzed and rerouted. In a polling system, a common return transmission path services both polling service requests and impending data transfers.
The VSAT polling technique entails a complex and costly hub. The technique is set up to accommodate many short bursts of return site data. The polling methodology is not conducive to provide timely responses in a consistent manner to the remote users. It also does not provide for deterministic status monitoring and control on a continuous basis of the remote terminals.
Another technique, demand assigned multiple access (DAMA), routes data between ground stations without a requirement that it be transmitted to a central hub for redistribution with a second satellite transmission. This technology is more costly to implement over a large number of remote sites than a polled architecture; however, it supports larger bandwidths. If available, a transmit channel is temporarily assigned to the remote site via a transmission from one of the other sites in the network. The channels could be assigned by the site sending the transmission or assigned from an available pool by any site in the network. Thus, the DAMA technique, as opposed to the polling technique, does not require remote transmissions to go to one central hub station for retransmission to the destination station. It has been used, for example, in offering digital phone and data service in developing countries. With this technology, routing and switching are grouped into small earth station clusters, then sent to a main earth station where they are transmitted to the end destination. Specific application dependent pieces of earth station hardware perform the phone switching.
DAMA and polled satellite communication systems often use a TDMA (Time Division Multiple Access) technique. The TDMA technique enables several remote sites to share a common channel on a transponder of the communication satellite. That is, several remote sites will each transmit a fixed length transmission in a predetermined time sequence. Each transmission will address the specific channel at a precise time.
In most VSAT systems (Polled or DAMA) a remote satellite transmission contains both data information and status and control information. Because of this, neither type of satellite communication system achieves a precise predetermined or real time deterministic satellite transmission response from a remote terminal station.
Low Earth Orbital Satellites (LEOs)
LEOs operate both as single satellites and as part of a constellation of many satellites. LEOs operate in assigned non-geostationary polar orbital slots that have a much lower altitude than GEO's. GEO's operate at an altitude of 22,300 miles above earth whereas LEOs can operate at altitudes from 375 to 1,250 miles. Like GEO's, some LEOs are planned to operate with comparable size transponders (36 MHz and 54 MHZ transponders). GEO's often have 24 transponders and LEOs often have less than one half that number. Fewer transponders provide less total satellite bandwidth.
It takes many non-geostationary LEOs operating in a polar arc above earth to fill the needs of a single geostationary earth station operating 24 hours a day. This is due to the need to have multiple LEOs because of the limited time each LEO physically has in orbit while over a specific ground station. LEOs are placed in a polar arc and operate at a fixed distance from each other. Therefore, a specific LEO satellite passing over an earth station is accessible by the earth station for a finite period of time. The earth station would therefore be dependent on communicating with many LEOs over a 24-hour period in order to maintain continuous communication. LEO earth stations addressing the low data rate mobile telephone market can use omni-directional antennas to send and receive transmissions from the satellite presently orbiting above its ground station. LEOs addressing the medium or high-speed data transfer market require the earth stations to use one or more mechanical tracking antennas. A new antenna technology referred to as a phased array antenna is being developed by others such as Motorola to reduce the cost of an earth station that communicates with LEOs.
Satellite to satellite transmissions between several LEOs orbiting in the same polar arc are performed by using a processing technique called store and forward. LEOs have an onboard microprocessor that can perform limited processing functions. The processor can analyze a special packet header on a data packet and cause the data packet to be forwarded to an adjacent satellite in the polar arc. Special packet headers contained in the front of data packets are used for routing the packets between satellites in the polar arc. Once a packet is accepted for processing by its intended remote earth station site, the packet is processed in a similar manner as a packet sent to a remote earth station site from a geostationary satellite.
A LEO can operate in a message/broadcast or Interactive mode of transmission. A message/broadcast mode sends packets to all ground station locations simultaneously (point to multipoint); an interactive mode is point to point.
Satellite to satellite transmissions occur via a Ka band or V band spectrum. Future developments may replace these links with laser communication technology. The present invention is not dependent on either type of satellite to satellite transmission technology. LEOs supporting voice, interactive multimedia and data applications usually communicate to earth stations via the Ka or V band spectrum.
LEOs are divided into three categories: Micro LEOs, Mid LEOs, and Big LEOs. The Micro, Mid and Big usually define the physical size of the satellite and its overall bandwidth capacity and the amount of earth's surface covered by its network of satellites. All three types operate within the altitude envelope above earth of 375 to 1,250 miles. If LEOs try to operate below 375 miles, they become susceptible to the earth's gravity. Most LEOs operate at an altitude of between 435 and 800 miles.
Big and Mid-size LEOs are being deployed and licensed for global communications. Micro LEOs are being deployed and licensed primarily for regional communications. Most United States Federal Communications Commission (FCC) licenses being granted are for global networks. The first global LEO satellite network to reach operational status is used to serve the mobile phone market. Medium and big LEOs are usually capable of receiving transmissions from high bandwidth transmissions (partial T1 and full T1). LEOs usually operate by using one of two techniques FDMA/TDMA, or CDMA. LEOs operating with Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) techniques are applicable to this invention.
LEOs operating with a Code Division Multiple Access (CDMA) technique probably are not applicable to the present invention due to their lower and limited bandwidth capabilities. CDMA is a technique in which a remote earth station uses a spread spectrum modulation and orthogonal codes to avoid interfering with other remote earth stations.
None of the previous GEO/LEO techniques discussed have provided a relatively simple and inexpensive way of communicating high-speed information that has a time dependency to and from remote site locations, especially the ones in which a high bandwidth connectivity infrastructure is not available to provide the high-speed information to and from the remote locations. Thus, there is the need for an improved and new satellite communication system and transmission method to satisfy this need.